Travelling wave tube device



April 1959 HIDEHIKO NlSHlO ETAL 2,882,439

' TRAVELLING WAVE TUBE DEVICE Filed Sep t. 19, 1955 l 1/ [III A Home y Un Sta es Pa se 2,882,439 TRAVELLING WAVE TUBE DEVICE Ilidehiko Nishio and Susumu Yasuda, Minato-ku, Tokyo,

Japan, assignors to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware This invention relates to travelling wave tubes and ,similar beam type tubes. I

In an electron tube, such as a travelling wave tube and the like, previous structures have provided a focussing field to maintain the electron beam emissioned from electron gun within an approximate unified diameter throughout in the long running path.

However, it is known that this focussing field need not be uniform and it is advantageous that magnetic induction field on the axis of the electron tube, such as the travelling wave tube and the like, to be varied periodically along its length, tor instance, with a sine wave pattern.

A principal advantage of having the externally applied field such an alternating field is that travelling-wave tube device can be considerably reduced in size in comparison with the uniform field type which has been used widely up to this time. However, when it is used combined with the wave guide type travelling wave tube, a discontinuity is produced in the magnetic circuit due to the electromag netic waveguide input and output couplers for the tube so that formation of the electron beam which having a desired unified diameter at these coupling points is diflicult.

To avoid these faults, it is desirable to achieve a distribution of a magnetic fiux over the spaced portion occupied by the waveguide by installing the pole pieces on both sides of the electromagnetic waveguide.

However, as the period of the field to be changed periodically with respect to the distance on the tube axis is preferred in general to be smaller than or at least no greater than the longitudinal spacing of the waveguide walls, the spacing of the pole piece at the waveguide should be as small as possible to avoid a discontinuous field.

Now let us make further explanation in regard to the period of alternating field as follows: In the case where the magnetic lens system produced by the alternating field has a large thickness, the condition that electron beam of the tube does not diverge would be given by following relative equation:

where e and m are an electric charge and a mass respectively of an electron; B is a maximum magnetic induction of alternating field on the axis; B is a helix electrode voltage; L is the period all in the metric system.

In the above equation, only the period L can be controlled, for e and m each have a fixed value, and B and V have definite values fixed by the particular tube used, and moreover the highest value of L is limited from the aforesaid equation.

On the other hand, it is desirable that the spacing of the pole pieces installed in line between to the electromagnetic waveguide be a half of the above-mentioned period for the alternating field to be continuous at the open space portion of the electro-magnetic waveguide.

However, since the width of the electromagnetic waveguide used generally is larger than a half of the period L, which is fixed with respect to the ordinary tube, the abovementioned arrangement wherein the spacing of the pole piece is made as large as the width of the electromagnetic waveguide, is not desirable for the desired electron beam focussing efiect.

The present invention avoids from such faults and, moreover, simplifies the configuration of the tube device and is characterized in that the electromagnetic waveguide itself is made of iron or other substance having a large permeability so that it provides an advantage that special pole piece for producing of the field is not'always necessary. v j

A better understanding of the invention may be had from the description of a particular example made with reference to the accompanying drawing, the single'figure' of which shows a fragmentary view, partly in section of a" travelling wave tube and waveguide structure. -In this line, 1 is a travelling wave tube, 2 are cylindrical magnets and each of them is so placed as their magnetic pole having a same symbol to oppose that of the neighbouring one. In this case the said magnets 2 may be either permanent magnets or electromagnets. Numeral 3 indicates iron yokes which are combined to the magnets 2 so that magnitude in the axial direction of the magnetic field on the tube axis is so distributed along the axis as to form an approximate sine wave pattern. Numeral 4 indicates an electromagnetic waveguide for power input or output. The wallplates. 5 perpendicular to the tube axis of the tube are made of a ferromagnetic substance and provided with the projections 6 which per forms the role of a magnetic shorting plate. These magnetic shorting plates 6 may be shaped as desired and need not be integral with the electromagnetic waveguide, however, the unitary construction provides the advantage of simplification of structure and effective utilization of space. Moreover, the wallplate 5 is not necessarily constructed completely of ferromagnetic material. From a practical view point it is desirable to provide a volume of magnetic material suflicient to assure that the magnetic resistance may be ignored for practical use compared with that one of the open space. On the inner surface at the tube the electromagnetic waveguide may be coated with a good conductor material for prevention of electric wave loss.

Nonmagnetic substance, for example brass or copper, is used as the wall portion of the electromagnetic waveguide running parallel with the tube axis. A piston 7 of non-magnetic material is provided for matching the impedance between the tube and the input and output electromagnetic waveguide circuit. Magnets 8 shaped as desired are used to complete a magnetic circuit in cooperation with the wall-plates 5 so that a necessary magnetic flux distribution is made across the gap between the walls of the electromagnetic waveguide.

In such a travelling-wave tube device, a special magnetic pole piece is not necessary so that the structure can be simplified and the weight can be reduced and at the same time an exciting magnetic force can be minimized because of the electromagnetic waveguide gap becomes the minimum possible value.

Even though the period of the alternating field produced by the magnets 2 and the iron yokes 3 is small, the magnetic field having a period almost of this period could be produced to the electromagnetic waveguide gap portion.

While we have explained, above, the present invention in connection with a travelling wave tube device, having a waveguide type input and output, it is not limited to travelling wave tubes but may be applied to similar devices in which the focusing field is interrupted by input and output circuits serving to receive or emit micro-wave Patented Apr. 14, 1959 power-"orto match the impedance between the external circuits and the tube input.

The invention is not limited only to the case wherein the alternatingfield is used, but can-produce a unified, field through any Waveguide gap no matter what type of field'is used, in a simple manner.

What is claimed is:

1. A travelling wave tube comprising an envelope, means to project an electron beam along a given path within said envelope, helical transmission means mounted within said envelope and extending along said path for transmitting electric wave energy for interaction with said beam, a rectangular waveguide of substantially uniform means external of said envelope for electric wave energy transfer, said waveguide having walls transverse of said transmission means and walls parallel to said 'transr'nis sion means, and focussing means for said beam, comprisa series of magnet means aligned longitudinally of said beam and positioned with like poles adjacent one another, elements of magnetic material extending from said magnet means into close proximity with said beam for focussing said beam, said elements being spaced apart a distance less than the spacing of the longitudinally spaced walls of said wave guide, said transverse walls of said waveguide adjacent said beam being of material of high magnetic permeability to preserve the focus of the magnetic field within the space of said waveguide and said parallel walls being of non-magnetic material.

2. A traveling wave tube according to claim 1, further comprising portions of high magnetic permeability matth rial forming extensions of said transverse walls extending beyond the outer periphery of said magnetic means, in spaced parallel relation, whereby an eifective magnetic shunt of relatively low reluctance is provided across the dimensions throughout coupled with said transmission space between said transverse walls.

References Cited in the file of this patent UNITED STATES PATENTS 2,741,118 Wang -t Apr. 10, 1956 2,797,360 Rogers et al June 25, 1957 FOREIGN PATENTS 

